The objective of this work is the characterization of antibody genes by their identification and detailed mapping. This effort is proceeding along several paths. Heavy chain genetics is being pursued by identifying additional crossover events in the heavy chain gene cluster for the purpose of learning gene order and organization and by identifying and mapping new heavy chain structural genes and other nearby genes. Light chain genes and efforts to identify new genes by means of structural and serological studies with myeloma proteins. The chromosomal loci of the heavy and light chain gene clusters is being sought by mapping studies with Recombinant Inbred strains and by cytogenetic means using in situ hybridization. In addition, construction of the NZB x C58 Recombinant Inbred (NX8RI) strains is nearing completion, and initial characterization of these strains has revealed a new and interesting type of crossover within the H-2 major histocompatibility complex.